1. Field of the Invention
The present invention generally relates to an electronic still camera and, more particularly, is directed to a still video camera having a simplified arrangement in which an automatic focusing control can be carried out in a short period of time and in which a white balance can be automatically adjusted with ease in a short period of time.
2. Description of the Prior Art
A variety of so-called electronic still cameras have been developed to record a still image on a magnetic disc as an electrical video signal. This kind of electronic still camera must adjust its focus by moving a focusing ring of an image pickup lens or the like similarly to an ordinary still camera utilizing a halide film. Some of such proposed electronic still cameras employ an auto-focusing mechanism to automatically adjust the focus. According to the auto-focusing mechanism of the above conventional electronic still camera, a distance from a camera to an object is detected by some suitable method and a focusing ring of an image pickup lens is moved in accordance with the thus detected distance information; thereby, the focus is adjusted automatically.
In a video camera, a focusing information, used to execute the auto focusing control, is detected from an imager signal itself and the auto focusing control is carried out without detecting the distance from the camera to the object (see Japanese Laid-Open Patent Publication No. 57-208520). More specifically, when the focusing adjustment of the lens is appropriate, a contour of an object is picked up very clearly so that an imager signal contains a high band frequency component. When on the other hand the focusing adjustment of the lens is not proper, the contour of the object is blurred so that the imager signal does not contain the high band frequency component. For this reason, by detecting the amount of the high band component of the imager signal, it is possible to determine whether or not the present focus adjustment state is proper. Thus, the auto focus control is carried out.
Because the above-mentioned electronic still camera employs a solid state image pickup element to obtain an electrical image pickup signal similarly to the video camera, the above auto focus system in which the focusing information for carrying out the auto focusing control is detected from the imager signal itself and can be applied to the electronic still camera. If this auto focusing system is applied to the electronic still camera, then the distance from the camera to the object need not be detected, which can make the auto focusing mechanism more simplified as compared with that of the still camera utilizing the halide film.
Although power must be supplied from a battery device or the like in order for the auto focusing mechanism to move a focus adjusting lens constructing a lens device, it is requested that the power consumed to carry out the auto focus control must be reduced as much as possible in order to extend the duration of the battery device. Particularly, the still camera generally employs a smaller battery device than that of the video camera because it must be miniaturized so that, if the auto focusing control is constantly carried out similarly to the video camera, there is then the disadvantage that the duration of the battery is reduced considerably.
Further, a zoom lens is widely utilized as an image pickup lens of this kind of camera. If the zoom lens is utilized as the image pickup lens, then a focal length can be varied successively, thus making it possible to take a picture at an arbitrary angle of view from, for example, a wide angle lens to a telephoto lens.
In the case of the zoom lens, if the focal length is changed, then the focusing position also is changed. That is, if the focal length is changed under the condition such that the object is accurately just-focused before the focal length is changed without changing the distance from the camera to the object, it is frequently observed that the zoom lens cannot be focused on the object any longer. Generally in the zoom lens, in order to prevent it from becoming out of focus, a focus adjusting lens is moved bit by bit by a mechanical part such as a cam groove or the like when a focal length adjusting zoom ring is rotated, whereby the focusing position is corrected in unison with the change of the focal length; thus, the focusing position is corrected.
However, the correcting mechanism composed of mechanism parts such as a cam groove or the like for correcting the focusing position has a relatively complicated arrangement, which unavoidably makes the lens device arrangement more complicated. Also, there is then the disadvantage that the lens device (i.e., camera device) is increased in weight and in volume.
Further, if the focusing position is merely corrected, then it is to be noted that the lens device cannot be always focused accurately on the object after the focal length is adjusted. That is, if the focusing position is corrected when the lens device is not properly focused on the object prior to the adjustment of the focal length, then the lens device cannot be properly focused on the object. There is then the disadvantage that the focusing position must be adjusted in order to properly focus the lens device on the object independently of the correction of the focusing position.
Further, when the cameraman takes a picture by the above-mentioned conventional electronic still camera, the white balance must be adjusted similarly to the ordinary video camera. That is, the white balance must be adjusted in accordance with a color temperature of light which illuminates the object to be taken. The adjustment of the white balance is carried out by adjusting the levels of three primary color signals (red, green and blue color signals) constituting an imager signal. By way of example, a ratio between the levels of red color signal and of green color signal and a ratio between the levels of blue color signal and green color signal are respectively detected and the levels of the red and blue color signals are adjusted so that the above-mentioned respective ratios fall within predetermined ranges; thereby, the white balance is adjusted. Generally, the white balance is automatically adjusted as described in Japanese Laid-Open Patent Publication No. 2-194792.
In the case of the ordinary video camera, because pictures are taken successively and imager signals must be output successively, the white balance must be adjusted by constantly operating the white balance adjusting circuit so that the white balance is constantly adjusted well.
On the other hand, in the case of the electronic still camera, the cameraman takes a still picture at a desired shutter release opportunity. Therefore, it is sufficient that various characteristics such as white balance or the like are satisfactorily adjusted just before the cameraman takes a picture. Accordingly, the white balance adjusting circuit is operated only immediately before the cameraman takes a picture or only when a shutter button, for example, is half pressed, which can suppress an operation time of the white balance adjusting circuit to the minimum. As a consequence, the power consumption of the electronic still camera can be suppressed to the minimum.
However, if the white balance adjusting circuit is operated only before the cameraman takes a picture as described above, when an electronic viewfinder for color display is mounted, for example, on this electronic still camera and the object is displayed on this electronic viewfinder, an unnatural image of the object having improper white balance is continuously displayed on the electronic viewfinder until the shutter button is half pressed. Furthermore, whenever the cameraman takes a picture, the cameraman must always half press the shutter button to confirm the adjusted state of the white balance by the electronic viewfinder beforehand. As a consequence, it is troublesome for the cameraman to take a picture and the cameraman cannot take a picture readily.